In Alan Couchman, Daryl Jones, “Multiport Amplifiers for Ka-band Multi-beam Payloads with Amplitude/Phase Compensation”, Proceedings of the The 13th Ka and Broadband Communications Conference, Turin Italy, Sep. 24-26, 2007, pp. 527-534, there is described multiport amplifiers (MPAs) as a means for enhancing the flexibility of Ka-band multi-beam payloads. However, at Ka-band maintenance of tracking within the MPA becomes highly challenging. A solution is the use of feedback loops located at specific points within the MPA to detect tracking errors and provide compensation. Errors are detected through measurements at “null points” in the MPA output network, with zero power corresponding to accurate tracking. The feedback loops adjust the MPA phase/gains such that the levels at these points are maintained at zero. The scheme operates with a pilot signal for null detection injected at one of the MPA inputs. The signal may be generated onboard and would be located outside the normal traffic space ensuring that the scheme would be non-invasive.
Whilst this solution is very effective at maintaining isolation, nevertheless it employs a significant amount of onboard hardware and on-board processing requirements. Since such requirements are at a premium in telecommunication satellites, further improvements in maintaining isolation within MPAs may be desirable.
An MPA is a well-known power amplifier device used for satellite communications, which operates at the microwave frequency bands. An MPA includes a number N of similar amplifier units (TWT or solid state) in parallel, each having a power P, so that each input signal is amplified equally by each amplifier, to potentially increase the power of an output signal by a factor N, to P×N. N input ports and N output ports are provided, so that an input signal on one input port is routed to the corresponding output port. The input ports are connected to the amplifier units by a low power input network (INET) that may be implemented in any convenient transmission line technology that is appropriate to the circumstances, e.g. microstrip, stripline, coaxial cable, or waveguide. The output ports are connected to the amplifier units by a high power output network (ONET) that is implemented typically using low loss transmission line technology. The ONET is mathematically a reciprocal of the INET, so that a signal presented to the nth input is directed to the nth output. Each network comprises an array of signal dividing waveguide devices. Butler matrices or networks comprising just hybrid devices are normally used for signal division, because they have convenient gain and phase shift properties. A hybrid is a four port signal dividing device comprising two inputs and two outputs, with selective 90° phase shifts; this phase difference may be exploited to improve the isolation characteristics of the networks. However other hybrids and other signal splitting devices may be used which may have 180° phase difference.
The great advantage of an MPA is that in providing access for each input port equally to each amplifier, the accessible power available to each port is N×P, where P is the power of each individual amplifier. Thus the MPA embodies a high degree of flexibility, providing a wide range of output power which can be shared dynamically and in a highly flexible manner between the N inputs (or downlink beams). However a concomitant problem with an MPA is that of cross-talk between MPA output ports, and in general a lack of isolation between signals routed through the MPA.